To read any data from ARconnect we have special interface
which includes two AUX registers: MCIP_CMD and MCIP_READBACK.
We write command to MCIP_CMD and read data from MCIP_READBACK
after that.

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We have only one instance of this registers per cluster, so we need
to held global lock before access them. This lock is defined
in arch/arc/kernel/mcip.c
To read GFRC value we also use MCIP_CMD/MCIP_READBACK pair, but
we take only local lock instead of global 'mcip_lock' lock:
---------->[drivers/clocksource/arc_timer.c]<----------
static u64 arc_read_gfrc(struct clocksource *cs)
{
unsigned long flags;
u32 l, h;
local_irq_save(flags);
__mcip_cmd(CMD_GFRC_READ_LO, 0);
l = read_aux_reg(ARC_REG_MCIP_READBACK);
__mcip_cmd(CMD_GFRC_READ_HI, 0);
h = read_aux_reg(ARC_REG_MCIP_READBACK);
local_irq_restore(flags);
return (((u64)h) << 32) | l;
}
-------------------------->8---------------------------
So we can break any command (like inter core interrupt send)
which uses MCIP_CMD/MCIP_READBACK pair when we read time from GFRC.
One possible solution is to create function like gfrc_read() in mcip.c
which will use global 'mcip_lock' and call it from current 'arc_read_gfrc'
function.
Or we can create a wrapper like 'mcip_read' in arch/arc/kernel/mcip.c
with next functionality:
------->8--------
u32 mcip_read(u32 cmd)
{
u32 ret;
raw_spin_lock_irqsave(&mcip_lock);
__mcip_cmd(cmd, 0);
ret = read_aux_reg(ARC_REG_MCIP_READBACK);
raw_spin_unlock_irqrestore(&mcip_lock);
return ret;
}
------->8--------
--
Eugeniy Paltsev
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